Transistor beta test and display circuit



Nov. 3, 1970 w. PAI YEN MAO TRANSISTOR BETA TEST AND DISPLAY CIRCUITFiled July 21, 1967 INVENTOR. T WILLIAM R Y. MAO

United States Patent 3,538,438 TRANSISTOR BETA TEST AND DISPLAY CIRCUITWilliam Pai Yen Mao, Southfield, Mich., assignor to BurroughsCorporation, Detroit, Mich, a corporation of Michigan Filed July 21,1967, Ser. No. 655,041 Int. Cl. Gtilr 31/22 11.5. Cl. 324-158 4 ClaimsABSTRACT OF THE DISCLOSURE An electronic circuit is disclosed fortesting the DC common emitter current gain of a transistor at a constantcollector to emitter voltage and providing an oscilloscope display ofthe current gain over a wide range of emitter currents. In this circuit,a saw-tooth shape current pulse is applied to the emitter of thetransistor under test and to the base through an operational amplifier.The emitter and base currents are then monitored and converted tologarithmic functions by semiconductor elements. These two logarithmicfunctions are subtracted to perform a division of the two currents andthe resulting voltage which indicates the log (hm-H) is applied to thevertical input of the oscilloscope. At the same time, the logarithm ofemitter current is applied to the horizontal input of the oscilloscope.

BACKGROUND OF *INVENTION V This invention relates to a method of andapparatus for testing the forward current gain of a transistor. As partof the invention, electronic analog circuits are utilized for producinga voltage proportional to the quotient of two measured currents.

Because manufactured transistors have different characteristics, evenamong those produced in the same batch, it is common for persons usingtransistors in manufactured equipment to perform a test upon eachtransistor prior to its installation. Such testing is necessary todetermine whether a given transistor has the required characteristicsfor use in a particular circuit. Since the current gain of a transistoris dependent upon the absolute current magnitude applied to it, it isdesirable to determine the current gain at many different currentlevels. Presently available testers do not provide for directlydisplaying the current gain of a transistor over its full operatingrange.

Prior art testing devices can generally be classified in one of twogroups. First, there are those devices which test the forward currentgain characteristics of a transistor at a single operating point. Thesetesters typically have fixed current sources and the collector toemitter voltage is used to indicate whether the current gain is largerthan the ratio of collector to base current. Or, they may have aconstant collector-to-emitter voltage and the base current is adjustedautomatically or manually and then the current gain is calculated foreach specific collector current condition. Since a transistor is seldomused at a single operating point, such a test serves only to get ageneral idea of the characteristics over the full operating range of thetransistor or the test must be repeated many times at differentoperating points.

The second group of prior art devices produces an oscilloscope displayof the volt-ampere characteristics over the full operating range of thetransistor under test. These devices still have the disadvantage thatfurther calculations are necessary to arrive at the forward current gainparameters. A reading of the currents and calculation of the currentgain must be made at many operating points. A device for directlydisplaying the transistor characteristics over its full operating range,on the other hand, will immediately tell the operator what he desires toknow concerning the transistor under test. This eliminates timeconsuming and possibly erroneous calculations.

One reason that prior art testing devices do not show current gain overthe full operating range of the transistor is that the electroniccalculating art does not teach a simple and accurate way for sensing twocurrents and dividing one by the other. The invention of such atechnique was necessary in order to effect a display of transistorcharacteristics over a full operating range.

In addition to these difficulties in using presently available testers,many prior art display devices require two varying voltage sources,which must be operated in synchronism, one for each of two elements ofthe transistor under test. This makes the testing circuit complicatedand subject to error. For simplicity and accuracy, a circuit requiringonly a single varying external voltage source is preferable.

All testers for measuring the transistor characteristics under steadystate conditions have the disadvantage of heating the transistor. As thetransistor temperature goes up its amplifying characteristics becomedistorted. In such testers, then, the range of currents applied duringthe test must be limited, thereby having the disadvantage of not beingable to measure the characteristics of the transistor over its fulloperating range.

SUMMARY OF INVENTION The present invention has overcome all theabove-mentioned disadvantages of the various prior art testing methodsand apparatus. The forward current gain characteristics of a transistorunder test are displayed on an oscilloscope or similar indicating devicefor the entire operating range of the transistor. A differentialamplifier supplies a current to a first transistor element in the amountrequired as a result of the transistor current gain characteristics, inresponse to a single variable external voltage source applied to asecond element of the transistor. The external variable voltage may beset to test the transistor over its full operating range. The inventionincludes means for sensing these two currents so supplied, and producingvoltages proportional to the natural logarithm of the currents. Furthermeans for subtracting one of these voltages from the other to effect adivision of the currents is provided. The resulting voltage is displayedon an oscilloscope or similar device with a logarithmic scale, therebyeffecting the taking of an antilogarithm. The result read is the ratioof two currents which is proportional to the forward current gain of thetransistor. This current ratio may be visually displayed or otherwisedetected for all operating points of the transistor, allowing theoperator to determine the desired information very quickly.

The aforementioned differential amplifier and transistor cooperate in anovel manner to keep the second transistor element at substantiallyground potential over the full operating range of the transistor. Aconstant voltage is applied to a third element of the transistorrelative to ground. The result is a constant voltage between the secondand third elements for the full range of currents to which thetransistor is subjected. Further, this circuit concept requires only asingle controlled voltage to supply the currents necessary for operationof the transistor.

It is desirable, in order to effectively display the information on anoscilloscope, that the external varying input voltage be a sawtooth-likewaveform. This voltage may be of a low duty cycle without impairing theoscilloscope display, thus allowing the junction temperature of thetransistors under test to remain substantially unaffected by the testcurrent.

Each of the currents of the transistor elements to be measured is sensedand converted to a voltage that is a natural logarithm of these currentsby means of a semiconductor junction element and detected by adifferential type of operational amplifier. These two voltages arearithmetically combined by means of an operational amplifier summationcircuit. Also injected into the summation circuit is a constantcontrolled voltage to compensate for the differences in thecharacteristics of the two semiconductor junction elements used fortaking the logarithm of the two currents measured. Compensation fordifferences in the current measuring semiconductor elements is necessaryfor accurate mathematical operations. This invention includes anuncomplicated circuit that performs logarithmic mathematical operationsas if its two current sensing semiconductor elements had exactly thesame voltampere characteristics.

BRIEF DESCRIPTION OF THE DRAWING The drawing discloses a preferredcircuit embodiment of the invention and shows individual apparatusblocks in dotted outline for performing the mathematical manipulation.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of theinvention as shown in the drawing is designed to measure the commonemitter forward current gain of a transistor under test. This transistorparameter is displayed on the vertical of an oscilloscope with thetransistors emitter current being applied in logarithmic form to thehorizontal of the oscilloscope. The common emitter forward current gainof a transistor, denoted as [3 or Ji is defined as the collector current(I divided by base current (1 with the collector voltage held constant.For reasons hereafter explained, in the embodiment shown in the drawingthe emitter current (I is measured instead of I This is still useful fordetermining the desired current gain characteristics since the ratio I/I equal lz -i-l. This is derived by combining the 11 definition and thetransistor current balance characteristic equation, I =I +I Referring tothe drawing, the circuit will be described generally. Differential oroperational amplifier 11 supplies base current to the transistor 13under test responsive to the external voltage V applied at terminals 30and 32. Block 21 senses the emitter current supplied from this externalsource, functions to produce an output voltage V proportional to thenatural logarithm of the emitter current, and applies this voltage tothe horizontal inputs 45 and 47 of oscilloscope 69. Block senses thebase current and produces an output voltage V, which is proportional tothe natural logarithm of the base current. Block 27 combines thevoltages V and V taking the difference between these voltages, andapplies its output V to the vertical inputs 41 and 43 of theoscilloscope 69. Voltage V is applied to block 27 to compensate for thedifference in logarithmic conversion characteristics of blocks 21 and15. The resulting oscilloscope display is the natural logarithm of (h+1) displayed along the vertical against the natural logarithm of Ialong the horizontal. The antilogarithm may be taken by appropriatescales on the oscilloscope tube. The tester operator has displayedbefore him the forward current gain for all operating points of thetransistor under test and may thereby make a quick choice as to whetherthe transistor will operate properly in the circuit being built.

The embodiment of the invention shown in the drawing for testing thecharacteristics of NPN transistors will now be explained in more detail.The output of the differential amplifier 11 supplies current throughdiode 19 to the base of the transistor under test 13. The emitter oftransistor 13 is connected to one input of amplifier 11. A variablevoltage source connected between terminals 30 and 32 creates a currentthrough R to the same input of amplifier 11, designated terminal 71. Thesecond input of amplifier 11 is placed at ground potential. The emittercurrent of transistor 13, then, is equal to V /R To understand theoperation of amplifier 11, consider input voltage V increasing from onelevel to another which will in turn place terminal 71 for a shortinstant at some voltage away from ground potential. As this happens, theoutput of amplifier 11 will increase since the voltage between itsinputs is increasing. The increased base current applied to transistor13 will cause the emitter current to increase to a steady state valuewhich depends upon the characteristics of transistor 13. As the emittercurrent so increases, the voltage drop across R will increase, therebydriving the voltage of terminal 71 again very near ground potential. Itis desirable that amplifier 11 have a very large gain so the inputdifference voltage for all operating levels of transistor 13 may be verysmall but still provide the required base current at the output ofamplifier 11.

If amplifier 11 is also chosen to have a very fast response time, V witha sawtooth waveform will provide required currents for both the emitterand the base of the transistor under test. A typical V will be asawtooth pulse width of 500 microseconds and a duty cycle of 2%, chosenfor low average current to minimize transistor heating. A separate basesupply, as required in many prior art display devices, is not necessary.Further, since terminal 71 is kept substantially at ground potential andterminal 35, the positive collector terminal as explained hereinafter,is kept at a constant voltage in relation to ground, thecollector-to-emitter voltage remains substantially constant While thefull range of operating currents is applied to the transistor.

From this basic transistor supply circuit the base and emitter currentsare independently measured and electronically divided for application tothe oscilloscope display unit. The base current is sensed and convertedto a logarithmic function by placing a semiconductor element, such asdiode 19 poled with its anode toward the base of transistor 13, inseries with the base with the voltage drop across the semiconductorelement providing the desired logarithmic function. As one ordinarilyskilled in the art will recognize, the equation relating the currentthrough and voltage drop across an ideal semiconductor diode is:

when V is greater than 0.1 volt,

Where:

la=the Boltzmann constant, 1.38X10 ergs per degree Kelvin T absolutetemperature, degrees Kelvin q=electronic charge, 1.602 l0 coulomb n anempirical factor near unity, dependent upon the semiconductor junctioncharacteristics.

If we let a=nkT q we see that:

( d Id ln sat) Therefore, the voltage across diode 19 equals:

Differential amplifier 17 transfers this voltage drop across diode 19 toa voltage between terminal 39 and ground denoted as V without loadingdown diode 19. The two inputs of amplifier 17 are connected on eitherside of diode 19 through equal resistors R The feedback resistor Rconnected between the output of amplifier 17 and one of its inputsdetermines the amplification factor of circuit 15. Since the realpurpose of circuit 15 is to sense the voltage drop across diode 19 andcreate an equal output voltage in relation to ground potential, R willgenerally be equal to R in order that the gain of the circuit shall beunity. Feedback resistor R is connected to the input of amplifier 17that will result in the output voltage at terminal 39 being the desiredpolarity.

d19 19( rvsat19) R is connected between the other input of amplifier 17and ground. With unity gain, the output voltage becomes:

The circuit within block 21 serves to produce an output voltage V whichis proportional to the natural logarithm of the emitter current.Differential amplifier 23 has one input placed at ground potential andoperates in cooperation with diode 25 in its feedback loop to keep theother input terminal 73 substantially at ground potential. Therefore,the input current to terminal 73 and thus through feedback diode 25 isequal to V /R Thus, the input current to this circuit is related to theemitter current of transistor 13 according to the ratio R /R Based onthe ideal diode characteristics expressed in Equation 1, it will be seenthat:

where K is the ratio R /R In order to keep diode 25 from heating, thecurrent through it is controlled to a fraction of I by making R severaltimes the resistance value of R In the embodiment presently beingdescribed, I is measured instead of I since the measurement of I maydisturb the constant voltage characteristic between terminals 35 and 71over the entire operating range of transistor 13. A diode in series withthe collector would create a variable voltage drop resulting in thecollector to emitter voltage being a function of the collector current,an undesirable characteristic sought to be eliminated. I is sensed bythe particular circuit configuration of block 21 in order to avoidplacing the diode in series between terminals 30 and 71 which wouldplace a further unknown impedance in the circuit and result ininaccuracies in the resulting measurements.

Block 27 combines the output voltages of blocks 15 and 21 along withthat of a constant voltage source impressed at terminals 31 and 33. Thevoltages V V and V are combined in an operational amplifier summationcircuit which is well known in the art. This summation circuit producesthe output voltage V The logarithmic function voltages V and V, areopposite in polarity so that their combination at point 77 results in asubtraction of these voltages and effects the desired division of theemitter and base currents of transistor 13. Feedback resistor R placedin the feedback loop of amplifier 29, along with the input resistors R Rand R will determine the total amplification of the individual voltagesapplied to block 27 for summation, in a manner well known in the art.

An important aspect of the present invention is the application of Vbetween terminals 31 and 33 in block 27. The purpose of introducing thisvoltage is to compensate for the difference in characteristics of diodes19 and 25 so that a subtraction of the logarithmic voltages produced bythese diodes results in accurate and meaningful information. How this isaccomplished and the value that e= rs( rasat-19) V should take may bestbe demonstrated by examiningthe basic mathematical relationships whichthe present circuit is designed to implement.

Recall, as developed hereinbefore:

( 4= 19( B sat-19) (5) V :a (ln I ln I ln K) It can be seen from thedrawing that:

(6) v z-R /R V R /R V, R,/R., V

substituting Equations 4 and 5 into 6,

( 1= 1/ s 25 IE'1I1 ent-25 K) In order to eliminate variations among theterms so they may be combined, the relative resistor values of block 27are made as follows:

R7/R6=1/(Z25; R /R =1/a R7/R4=1 substituting and regrouping, we have,

( Vlzln 13- E+ sat25 sat-+ 3 Therefore, the voltage applied in terminals31 and 33 in order to cancel unwanted terms, is,

and the voltage applied to the oscilloscope by substituting Equation 10in Equation 9 becomes,

The desired function.

Note by Equations 9 and 10 that V compensates for the dilferences insaturation currents (I a constant that may vary between the two diodesused in the circuit. The ratio of feedback to input resistances in thesummation circuit of block 27 compensates for the different valuesbetween diodes 19 and 25 of the exponential function of the diodecharacteristic Equation 1, expressed as the quantities a and ri inEquations 4 and 5. It should be noted that this quantity a is somewhattemperature dependent so compensating resistors R and R may be leftslightly variable for further adjustment under varying testingconditions. It is readily seen, then, that insertion of the constantvoltage V and adjustment of the relative values of the resistances inblock 27 compensates for any variations of V and V that might be due toa difference in characteristics between diodes 19 and 25. Therefore,block 27 produces an output V which is proportional to the quotient of Iand I and undistorted by such diode differences.

To further assure accuracy of the described circuit, differentialamplifiers 11, 17, 23, and 29 must be capable of responding to voltagechanges at their inputs without distortion. This requires reasonablyfast amplifiers which are also stable and have a wide bandwidth. Manysuch amplifiers are commercial available. However, many amplifiers thatsatisfy these requirements have a relatively low current output. Incertain testing circumstances, it may be desirable to increase thecurrent output of the operational amplifiers. One such means foraccomplishing this would be to add a simple emitter follower circuit atthe output of the amplifiers to increase their current drivingcapability.

The preceding detailed discussion of the preferred embodiment ofapplicants invention has described the circuit for testing thecharacteristics of an NPN transistor. Based on this description itshould be obvious to those ordinarily skilled in the art that certainmodifications may be made in order to test characteristics of PNPtransistors and obtain the same beneficial results over present testingcircuits. The external varying voltage V instead of being negativepolarity as shown in the drawing would be made a positive polarity todrive the current in the proper direction for the PNP transistor 13. Thepolarity of the collector voltage V as applied to terminals 35 and 37would also have to be reversed. In block 15, diode 19 would have itspolarity reversed, and in block 21, the polarity of diode 25 would bereversed. The applied voltage V in block 27 would also have its polarityreversed. The result will be that the voltage applied to theoscilloscope will have opposite polarities than in the NPN case.

The embodiment hereinabove described is the preferred way to practiceapplicants invention. Other embodiments and modifications may alsobecome apparent to those skilled in the art. For instance, a steadyvoltage may be desired for V in order to determine the amplification ofthe transistor under test at a single operating point. Although there isthe danger of overheating the semiconductor junction of the testtransistor, applicants invention would still provide the otheradvantages hereinabove de- 7 scribed which are not found in presenttechniques and apparatus.

Another useful modification of the preferred embodiment may include acircuit for taking the antilogarithm of V before application to thehorizontal input of the oscilloscope. This would result in anoscilloscope display with a linear instead of a logarithmic scale.

Also, it is possible to use a meter in place of the 0scilloscope forsteady state testing where V is manually controlled. As a furthermodification, an antilogarithmic device could be inserted between block27 and this meter to allow use of a linear meter scale.

Many variations in particular circuitry and types of elements to carryout this invention are possible without deviating from the true spiritand scope of this significant advance in the art as defined in thefollowing claims.

I claim:

1. An electronic circuit for determining the characteristics of anelectronic device with at least three elements such as a transistor,comprising:

means for applying a constant voltage in relation to ground potential toa first element of said device; means for applying a periodic sawtoothwaveform current to a second element of said device; differentialamplifier means responsive to said sawtooth waveform current forsupplying the necessary current to a third element of said device tokeep the device in current balance; said differential amplifier meanscooperating with said device to keep said second element substantiallyat ground potential;

an oscilloscope with a first and second input for displayingcharacteristics of said device;

means for converting said sawtooth waveform current into a voltageproportional to the natural logarithm of said current and applying saidvoltage to said first oscilloscope input;

means for converting the current of said second element into a voltageproportional to the natural logarithm of said current; and

means for arithmetically combining said voltage proportional to thecurrent of said second element with said voltage proportional to thecurrent of said third element and applying the combination to saidsecond oscilloscope input.

2. In an apparatus for testing the characteristics of a three or moreterminal electronic device by supplying current to a plurality of saidterminals and for applying signals indicative of said characteristics toa display device, the improvement comprising:

means including diodes for individually sensing the current at first andsecond terminals of said device and for converting each current into avoltage proportional to the logarithm of said current,

said sensing and converting means including a first ditferentialamplifier connected across one of said 8 diodes and a seconddifferential amplifier-having another of said diodes in a feedback loop,and

means for arithmetically combining said voltages to provide signalsindicative of said characteristics, said combining means including anoperational amplifier and means for compensating for the differences inthe characteristics of said diodes.

3. The apparatus of claim 2 wherein a varying current is supplied tosaid first terminal of said device and includmg:

means responsive to said varying current at said first terminal forautomatically providing a varying current to said second terminal ofsaid device for testing said device at'steady state.

said combining means thereby providing signal-s indicative ofsaidcharacteristics substantially throughout the operating range of saiddevice.

4. In an apparatus for-testing the characteristics of a three or moreterminal electronic device by supplying a varying current to said firstterminal of said device and for applying signal-s indicative of saidcharacteristics to a display device, the improvement comprising:

means responsive to said varying current at said first terminal forautomatically providing a varying current to said second terminal fortesting said device at steady state,

means including diodes for individualy sensing the current at first andsecond terminals of said device and for converting each current into avoltage proportional to the logarithm of said current,

said sensing and converting means including a first differentialamplifier connected across one of said diodes and a second differentialamplifier having another of said diodes in a feedback loop, and, meansfor arithmetically combining said voltages to provide signals indicativeof said characteristics.

References Cited UNITED STATES PATENTS 6/1941 Martin.

OTHER REFERENCES RUDOLPH V. ROLINEC, Primary Examiner E. L. STOLARUN,Assistant Examiner US. Cl. X.R.

